STUDIES ON THE REACTION MECHANISM OF THE FISCHER-TROPSCH SYNTHESIS ON IRON AND COBALT CATALYSTS

摘要

Although numerous studies over seventy years concern the mechanism of the F.-T. synthesis this subject still remains controversially. At present many authors favour the CH2 insertion mechanism as dominant for the F.-T. synthesis. However, the formation of oxygenates is hardly feasible via the CH2 insertion mechanism. Therefore, oxygenates are assumed to be formed via the CO insertion mechanism. In order to explain both the formation of hydrocarbons and oxygenates M.E. Dry [1] proposed a mechanism that involves both CH2 and CO as active surface intermediates. For all Fischer-Tropsch catalysts deviations from the ideal Anderson-Schulz-Flory (ASF) distribution are observed. For both iron and cobalt catalysts product distributions can be represented by superposition of two ASF distributions characterized by the growth probabilitiesα1, α2, and f2 the mass fraction of distribution 2 characterized byα2. The hypothesis that these deviations can be exclusively traced back to readsorption of 1-alkenes and secondary chain propagation and that the CH2 insertion mechanism is the exclusive one could be definitely excluded by means of stoichiometric calculations on the basis of 1-alkene and ethene co-feeding experiments. These experiments using a Co catalyst have shown that readsorbed 1-alkenes and ethene grow with the probability α1. Based on these 1-alkene and ethene co-feeding experiments with cobalt catalysts and on the promoter effect of alkali on iron catalysts we have drawn the conclusion that the two superimposed ASF distributions with different chain growth probabilities are the result of different chain growth mechanisms that are not compatible. The aim of the present study is to develop a consistent hypothesis of the mechanism of the F.-T. synthesis on the basis of co-feeding experiments with 1-alkenes, ethene, alcohols and CH2N2 as a source of CH2, and on the formation of branched hydrocarbons, alcohols and aldehydes. The formulation of the novel mechanism follows the knowledge of analogous reactions in homogeneous catalysis and should give a detailed insight in the crucial step of C-C linkage.